RAPID EXCHANGE OF SUBUNITS OF MAMMALIAN ORNITHINE DECARBOXYLASE

The subunit structure of mouse L-ornithine decarboxylase (ODC) was inv estigated using mutants involving single amino acid changes that great ly reduced the catalytic activity. Studies were carried out both by ex pressing the enzyme protein in a coupled transcription/translation sys tem and mixing the various purified mutant ODCs and wild type enzyme t ogether. The results confirm that ODC activity requires the formation of a dimer and that this dimer contains two active sites, each made up from part of one subunit that contains amino acids lysine 69, lysine 169, and histidine 197 and a part of the other subunit that contains c ysteine 360. Mixing of the purified ODC mutant enzymes with each other and with the wild type enzyme indicated that there was a very rapid e xchange of subunits between the enzyme dimers even under physiological conditions without addition of chaotropic agents. This rapid exchange may facilitate the binding of antizyme and the rapid turnover of ODC in vivo.